A method for correcting the phase modulation depth of a laser frequency modulated interferometer

Abstract

This invention discloses a method for correcting the phase modulation depth of a laser frequency-modulated interferometer, relating to the field of fiber optic interferometric displacement measurement technology. The method includes: outputting a first modulated laser and a second modulated laser from a frequency-modulated laser; the first modulated laser generates interference after passing through a beam splitter and is scanned and output by a first photodetector; the second modulated laser absorbs a specific wavelength of laser light through a specific absorption spectral line in a gas absorption cell, is detected by a second photodetector, and demodulated by a demodulation module; the scanning time of the two peaks of the demodulated laser intensity signal is obtained, and the total optical path difference of the interference signal is obtained by measuring the relative displacement of the measuring mirror and the initial optical path difference; the phase modulation depth is obtained by the scanning time and the total optical path difference; and the current amplitude of the frequency-modulated laser is adjusted by a modulation module. Phase modulation depth correction can be achieved by controlling the current output through the modulation module, reducing the difficulty of phase modulation depth correction.

Description

Technical Field

[0001] This invention relates to the field of fiber optic interferometric displacement measurement technology, and in particular to a method for correcting the phase modulation depth of a laser frequency modulated interferometer. Background Technology

[0002] Precision displacement measurement is a key technology in advanced manufacturing, and its accuracy directly affects the quality of production. Among numerous displacement measurement instruments, laser interferometers offer the highest precision. Over the decades, high-precision displacement interferometers such as single-frequency and dual-frequency interferometers have emerged, greatly promoting the development of high-precision manufacturing. Today, modern high-end equipment manufacturing places even stricter demands on interferometers, such as small size, ease of installation, and high robustness. Fiber optic sinusoidal frequency modulated interferometers (SFMI) provide a near-perfect solution for these requirements.

[0003] The Michelson laser interferometric displacement sensor is the most classic optical interferometric displacement measurement method. After long-term research and development, the technology is mature and reliable, and the resolution after circuit subdivision can reach 1nm. However, its measurement accuracy is directly related to the stability of the light source wavelength, so it has high requirements for the environment in which the light source and optical path are located. In addition, due to the existence of sinusoidal error, the measurement range is greatly limited.

[0004] Fiber optic sinusoidal frequency modulated interferometers, based on PGC technology, offer high sensitivity and excellent accuracy, making them widely used in vibration measurement and displacement sensing. However, the phase modulation depth of a sinusoidal frequency modulated interferometer changes with the position of the target object, and the linear measurement range for displacement / vibration is often only a few to tens of micrometers. This results in existing phase modulation depth correction algorithms having extremely high computational load and consuming excessive hardware resources, which greatly limits the application of sinusoidal frequency modulated interferometers. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing a method for correcting the phase modulation depth of a laser frequency modulated interferometer. This method solves the problem that the phase modulation depth of a sinusoidal frequency modulated interferometer changes with the position of the measurement target. For linear measurements of displacement / vibration, the range is often only a few to tens of micrometers, resulting in a huge computational load and excessive hardware resource consumption in existing phase modulation depth correction algorithms. This greatly limits the application of sinusoidal frequency modulated interferometers.

[0006] This invention specifically provides the following technical solution: a method for correcting the phase modulation depth of a laser frequency modulated interferometer, comprising the following steps:

[0007] The tunable laser of the laser frequency modulation interferometer outputs the first and second modulated lasers.

[0008] The first modulated laser is split into two beams by a beam splitter prism. The two laser beams are respectively sent to a reflector and a measuring mirror. The laser sent to the reflector is used as a reference beam and the laser sent to the measuring mirror is used as a measuring beam. When the reference beam and the measuring beam are reflected back, they interfere with each other. The interfering laser is detected by a first photodetector, and the signal detected by the first photodetector is demodulated by a demodulation module to obtain the interference signal.

[0009] The second modulated laser is input into a gas absorption cell. The second modulated laser of a specific wavelength is received through two specific absorption spectral lines in the gas absorption cell. The second modulated laser of a specific wavelength is detected by a second photodetector. The signal detected by the second photodetector is input into a demodulation module for demodulation to obtain the modulated laser intensity signal.

[0010] The scanning time corresponding to the two peaks of the modulated laser intensity signal is obtained, and the optical path from the beam splitter to the measuring mirror and the initial optical path difference between the beam splitter and the reflecting mirror are obtained. The total optical path difference of the interference signal is obtained based on the sum of the relative displacement of the measuring mirror and the initial optical path difference.

[0011] The phase modulation depth is obtained by the scanning time and the total optical path difference of the interference. When the phase modulation depth reaches the ideal value, the current amplitude of the frequency-tunable laser is adjusted by the modulation module. The phase modulation depth is corrected by the relationship between the current amplitude and the laser frequency.

[0012] Preferably, the step of demodulating the signal detected by the first photodetector using a demodulation module to obtain the interference signal includes the following steps:

[0013] The phase value of the current signal of the tunable laser is extracted using Hilbert transform, and the specific expression is as follows:

[0014]

[0015] Where I(t) is the current signal and t is the time variable. Let j be the Hilbert transform of I(t), where j is the imaginary unit. Let I(t) be the value after the Hilbert transform. for The phase angle, unwrap[] denotes the phase expansion by order;

[0016] The specific expression for the interference signal is as follows:

[0017]

[0018] Preferably, obtaining the initial optical path difference between the beam splitter prism and the measuring mirror, and between the beam splitter prism and the reflecting mirror, includes the following steps:

[0019] According to the dual-wavelength interference theory, the initial optical path difference is obtained, and the specific expression is as follows:

[0020]

[0021] Where l0 is the initial optical path difference, t1 and t2 are two different time points corresponding to the two signal peaks detected by the second detector, and λ1 and λ2 are the laser wavelengths corresponding to the two signal peaks detected by the second photodetector.

[0022] Preferably, the specific expression for the total optical path difference of the interference is l = l0 + l(t), where l(t) is the relative displacement of the measuring mirror.

[0023] Preferably, the phase modulation depth is obtained by the scanning time and the total optical path difference of the interference, and when the phase modulation depth reaches the ideal value, the current amplitude of the tunable laser is adjusted by the modulation module, and the phase modulation depth is corrected by the relationship between the current amplitude and the laser frequency, including the following steps:

[0024] Obtained from the phase modulation depth expression:

[0025] C=2πl(t)Δν(t) / c=2.63

[0026] Where C is the ideal value of phase modulation depth, c is the speed of light, and Δv is the frequency modulation depth;

[0027] The frequency modulation depth Δv is linearly transformed to obtain the current modulation depth ΔI, which is specifically expressed as follows:

[0028] ΔI=k iν Δν

[0029] Where, k iv These are constant coefficients, calibrated through offline data acquisition.

[0030] Preferably, before using the second photodetector to detect the second modulated laser of a specific wavelength, the operating temperature of the tunable laser is adjusted by the modulation module, and the wavelength of the output laser of the tunable laser is linearly scanned.

[0031] Preferably, when adjusting the operating temperature of the tunable laser by the modulation module and performing a linear wavelength scan of the output laser of the tunable laser, P10 and P11 in H13C14 are selected as two absorption spectral lines, and the wavelengths corresponding to the two absorption spectral lines are used as the measurement wavelengths.

[0032] Preferably, before splitting the first modulated laser beam into two beams by the beam splitter, the following steps are also included:

[0033] The first modulated laser is passed through an input fiber circulator along a single-mode fiber.

[0034] The laser light is focused into the collimator via the fiber optic circulator, and then the focused laser light is input into the beam splitter.

[0035] Preferably, when interference occurs when the reference beam and the measurement beam are reflected back, the reference beam and the measurement beam interfere when passing through a single-mode optical fiber.

[0036] Compared with the prior art, the present invention has the following significant advantages:

[0037] This invention outputs two modulated laser beams from a frequency-tunable laser. One beam passes through a beam splitter to generate interference and is scanned and output by a first photodetector. The other beam absorbs a specific wavelength of laser light through a specific absorption line in a gas absorption cell, is detected by a second photodetector, and is demodulated by a demodulation module. The scanning time of the two peaks of the demodulated laser intensity signal is obtained, and the total optical path difference of the interference signal is obtained by measuring the relative displacement of the measuring mirror and the initial optical path difference. The phase modulation depth is obtained by the scanning time and the total optical path difference. The current amplitude of the frequency-tunable laser is adjusted by the modulation module. The phase modulation depth can be corrected by controlling the current output through the modulation module. Only an additional signal is needed to obtain the total optical path difference of the interference and obtain the phase modulation depth by the scanning time and the total optical path difference. The frequency modulation amplitude can be linearly converted into the current modulation amplitude. The phase modulation depth can be corrected by controlling the current output through the modulation module. This invention solves the problem that the phase modulation depth correction algorithm has a huge computational load and consumes too many hardware resources when the linear measurement range of displacement / vibration is extremely small. It improves the correction effect and reduces the difficulty of phase modulation depth correction. Attached Figure Description

[0038] Figure 1 This is a structural schematic diagram provided in an embodiment of the present invention.

[0039] Among them, 1-tunable laser, 2-fiber circulator, 3-single-mode fiber, 4-collimator, 5-reflector, 6-measuring mirror, 7-beam splitter, 8-first photodetector, 9-gas absorption cell, 10-second photodetector, 11-demodulation module, 12-host computer, 13-modulation module. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0041] like Figure 1 As shown, the laser frequency-modulated interferometer structure includes: a frequency-tunable laser 1, an optical fiber circulator 2, a single-mode optical fiber 3, a collimator 4, a reflector 5, a measuring mirror 6, a beam splitter 7, a first photodetector 8, a demodulation module 11, a host computer 12, and a modulation module 13. A gas absorption cell 9 and a second photodetector 10 are added to correct the phase modulation depth.

[0042] The system comprises: a frequency-tunable laser 1 for outputting a first modulated laser and a second modulated laser; an fiber optic circulator 2 connected at its input to the frequency-tunable laser 1 and at its output to a collimator 4, which inputs the first modulated laser into the collimator 4; a beam splitter 7 connected to the collimator 4 to split the focused modulated laser into two beams, generating interference, which is then input into the fiber optic circulator 2; and a first photodetector 8 connected at its input to the fiber optic circulator 2 and at its output to a demodulation module 11. The modulation module 11 is connected to the host computer 12; the input end of the gas absorption cell 9 is connected to the tunable laser 1, and two specific absorption spectral lines are set in the gas absorption cell 9 to receive the second modulated laser of a specific wavelength; the input end of the second photodetector 10 is connected to the gas absorption cell 9, and the output end is connected to the demodulation module 11; the input end of the modulation module 13 is connected to the host computer 12, and the host computer 12 records the modulated laser light intensity signal, and the output end is connected to the tunable laser 1 to output the modulation current for the tunable laser 1 to perform phase modulation depth correction.

[0043] A single-mode fiber 3 is disposed between the fiber optic circulator 2 and the collimator 4. The single-mode fiber 3 is used to interfere with the two laser beams. The two laser beams include the laser beam reflected by the beam splitter 7 to the reflector 5 and the laser beam transmitted by the beam splitter 7 to the measuring mirror 6.

[0044] Based on the structure of a laser frequency-modulated interferometer and the added correction structure, this invention provides a method for correcting the phase modulation depth of a laser frequency-modulated interferometer. Utilizing the theoretical relationship between current modulation depth and absolute distance, the method initializes the current modulation depth at the operating point of the laser sinusoidal frequency-modulated interferometer, comprising the following steps:

[0045] Step S1: Output the first modulated laser and the second modulated laser through the tunable laser 1 of the laser frequency modulation interferometer.

[0046] Step S2: The first modulated laser is split into two beams by the beam splitter prism 7. The two laser beams are sent to the reflector 5 and the measuring mirror 6, respectively. The laser sent to the reflector 5 is used as the reference beam, and the laser sent to the measuring mirror 6 is used as the measuring beam. When the reference beam and the measuring beam are reflected back, interference occurs. The laser beam that causes interference is detected by the first photodetector 8, and the signal detected by the first photodetector 8 is demodulated by the demodulation module 11 to obtain the interference signal.

[0047] Before the first modulated laser beam is split into two beams by the beam splitter, the following steps are also included:

[0048] The first modulation path is input into the fiber circulator 2 along the single-mode fiber 3;

[0049] The laser light is focused by the collimator 4 through the fiber optic circulator 2, and then the focused laser light is input into the beam splitter 7.

[0050] The process of demodulating the signal detected by the first photodetector using a demodulation module to obtain the interference signal includes the following steps:

[0051] The phase value of the current signal of the frequency-tunable laser is extracted using Hilbert transform, and the specific expression is as follows:

[0052]

[0053] Where I(t) is the current signal and t is the time variable. The Hilbert transform of I(t) yields... phase angle j is the imaginary unit. Let I(t) be the result of the Hilbert transform, and unwrap[] denotes the phase expansion by order.

[0054] The specific expression for the interference signal is as follows:

[0055]

[0056] Step S3: Input the second modulated laser into the gas absorption cell 9, receive the second modulated laser of a specific wavelength through two specific absorption spectral lines in the gas absorption cell 9, use the second photodetector 10 to detect the second modulated laser of the specific wavelength, input the signal detected by the second photodetector 10 into the demodulation module 11 for demodulation, obtain the modulated laser light intensity signal, and transmit it to the host computer 12 for recording.

[0057] When the second photodetector 10 is used to detect the second modulated laser of a specific wavelength, the operating temperature of the laser is adjusted by the modulation module 13 to perform a linear wavelength scan of the laser output. The frequency-tunable laser performs a frequency scan by adjusting the temperature of the laser diode.

[0058] When performing frequency scanning by adjusting the temperature of the laser diode, the wavelengths corresponding to the P10 and P11 absorption lines of H13C14 are selected as the measurement wavelengths.

[0059] Step S5: Obtain the scanning time corresponding to the two peaks of the modulated laser intensity signal calculated by the host computer 12, and obtain the optical path from the beam splitter 7 to the measuring mirror 6, and the initial optical path difference between the beam splitter 7 and the reflecting mirror 5. Based on the sum of the relative displacement of the measuring mirror 6 and the initial optical path difference, obtain the total optical path difference of the interference signal.

[0060] The initial optical path difference between the beam splitter 7 and the measuring mirror 6, and between the beam splitter 7 and the reflecting mirror 5, includes the following steps:

[0061] According to the dual-wavelength interference theory, the initial optical path difference is obtained, and the specific expression is as follows:

[0062]

[0063] Where l0 is the initial optical path difference, t1 and t2 are two different time points corresponding to the two signal peaks detected by the second detector, and λ1 and λ2 are the laser wavelengths corresponding to the two signal peaks detected by the second photodetector 10.

[0064] The specific expression for the total optical path difference of the interference is l = l0 + l(t), where l(t) is the relative displacement.

[0065] Step S6: Obtain the phase modulation depth by scanning time and total optical path difference of interference, and when the phase modulation depth reaches the ideal value, adjust the current amplitude of the tunable laser 1 by modulation module 13, and perform phase modulation depth correction by the relationship between current amplitude and laser frequency.

[0066] The process involves modulating the laser injection current amplitude when the total optical path difference of the interference reaches a threshold. Based on the linear conversion relationship between frequency modulation amplitude and current modulation, the modulation module 13 controls the modulation current output to perform phase modulation depth correction, including the following steps:

[0067] Obtained from the phase modulation depth calculation formula:

[0068] C=2πl(t)Δν(t) / c=2.63

[0069] Where C is the ideal value of phase modulation depth, c is the speed of light, and Δv is the frequency modulation depth.

[0070] The frequency modulation depth Δv is linearly transformed to obtain the current modulation depth ΔI, which is specifically expressed as follows:

[0071] ΔI=k iν Δν

[0072] Where, k iv These are constant coefficients, calibrated through offline data acquisition.

[0073] The above description, in conjunction with specific preferred embodiments, provides a more detailed explanation of the present invention. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for correcting the phase modulation depth of a laser frequency-modulated interferometer, characterized in that, Includes the following steps: The tunable laser of the laser frequency modulation interferometer outputs the first and second modulated lasers. The first modulated laser is split into two beams by a beam splitter prism. The two laser beams are respectively sent to a reflector and a measuring mirror. The laser sent to the reflector is used as a reference beam and the laser sent to the measuring mirror is used as a measuring beam. When the reference beam and the measuring beam are reflected back, they interfere with each other. The interfering laser is detected by a first photodetector, and the signal detected by the first photodetector is demodulated by a demodulation module to obtain the interference signal. The second modulated laser is input into a gas absorption cell. The second modulated laser of a specific wavelength is received through two specific absorption spectral lines in the gas absorption cell. The second modulated laser of a specific wavelength is detected by a second photodetector. The signal detected by the second photodetector is input into a demodulation module for demodulation to obtain the modulated laser intensity signal. The scanning time corresponding to the two peaks of the modulated laser intensity signal is obtained, and the optical path from the beam splitter to the measuring mirror and the initial optical path difference between the beam splitter and the reflecting mirror are obtained. The total optical path difference of the interference signal is obtained based on the sum of the relative displacement of the measuring mirror and the initial optical path difference. The phase modulation depth is obtained by the scanning time and the total optical path difference of the interference. When the phase modulation depth reaches the ideal value, the current amplitude of the frequency-tunable laser is adjusted by the modulation module. The phase modulation depth is corrected by the relationship between the current amplitude and the laser frequency.

2. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 1, characterized in that, The step of demodulating the signal detected by the first photodetector using a demodulation module to obtain the interference signal includes the following steps: The phase value of the current signal of the tunable laser is extracted using Hilbert transform, and the specific expression is as follows: Where I(t) is the current signal and t is the time variable. Let j be the Hilbert transform of I(t), where j is the imaginary unit. Let I(t) be the value after the Hilbert transform. for The phase angle, unwrap[] denotes the phase expansion by order; The specific expression for the interference signal is as follows:

3. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 2, characterized in that, The process of obtaining the initial optical path difference between the beam splitter prism and the measuring mirror, and between the beam splitter prism and the reflecting mirror, includes the following steps: According to the dual-wavelength interference theory, the initial optical path difference is obtained, and the specific expression is as follows: Where l0 is the initial optical path difference, t1 and t2 are two different time points corresponding to the two signal peaks detected by the second detector, and λ1 and λ2 are the laser wavelengths corresponding to the two signal peaks detected by the second photodetector.

4. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 3, characterized in that, The specific expression for the total optical path difference of the interference is l = l0 + l(t), where l(t) is the relative displacement of the measuring mirror.

5. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 4, characterized in that, The phase modulation depth is obtained by the scanning time and the total optical path difference of the interference. When the phase modulation depth reaches the ideal value, the current amplitude of the tunable laser is adjusted by the modulation module. The phase modulation depth is corrected by the relationship between the current amplitude and the laser frequency, including the following steps: Obtained from the phase modulation depth expression: Where C is the ideal value of phase modulation depth, c is the speed of light, and Δv is the frequency modulation depth; The frequency modulation depth Δv is linearly transformed to obtain the current modulation depth ΔI, which is specifically expressed as follows: ΔI=k iν Dn Where, k iv These are constant coefficients, calibrated through offline data acquisition.

6. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 1, characterized in that, Before using the second photodetector to detect the second modulated laser of a specific wavelength, the operating temperature of the tunable laser is adjusted by the modulation module, and the wavelength of the output laser of the tunable laser is linearly scanned.

7. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 6, characterized in that, When adjusting the operating temperature of the tunable laser via the modulation module and performing a linear wavelength scan of the output laser, the wavelengths corresponding to the absorption lines P10 and P11 in H13C14 are selected as the measurement wavelengths.

8. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 1, characterized in that, Before splitting the first modulated laser beam into two beams using a beam splitter, the following steps are also included: The first modulated laser is passed through an input fiber circulator along a single-mode fiber. The laser light is focused into the collimator via the fiber optic circulator, and then the focused laser light is input into the beam splitter.

9. The method for correcting the phase modulation depth of a laser frequency modulated interferometer as described in claim 1, characterized in that, When interference occurs when the reference beam and the measurement beam are reflected back, interference also occurs when the reference beam and the measurement beam pass through a single-mode optical fiber.

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